JERUSALEM (AP)  Israeli helicopters attacked two suspected Hamas weapons workshops in Gaza City early Monday and Prime Minister Ariel Sharon called off a summit with his Palestinian counterpart — a first response to a double suicide bombing that killed 10 Israelis in a heavily guarded seaport.

Palestinian youths look at the damaged metal workshop targeted by Israeli helicopters in Gaza City early Monday.

By Mohammed Abed, AFP

Sunday's bombing in Ashdod port was the first deadly attack on a strategic target in Israel in more than three years of fighting and sent Israeli officials scrambling to discover security breaches.

A militant leader in Gaza told The Associated Press the bombers had intended to blow up fuel storage tanks in Ashdod, confirming Israeli suspicions that militants are trying to carry out a "mega attack" with hundreds of casualties. The explosions went off several hundred meters from the fuel tanks.

The bombers, 17-year-old high school students from a Gaza refugee camp, managed to slip into Israel despite a heavily patrolled fence ringing the strip; one of their handlers said he believed they crawled through a tunnel. The assailants also evaded tight security at Ashdod port and used high-grade plastic explosives.

The Islamic militant Hamas and the Al Aqsa Martyrs' Brigade, a group with ties to Yasser Arafat's Fatah movement, claimed joint responsibility.

The intended target, the use of plastic explosives, the cooperation between militant groups and the fact that Palestinian bombers managed to slip out of Gaza for the first time since 2000 were all seen as signals of escalation in tactics.

One Al Aqsa leader in Gaza, who identified himself only as Abu Qusay, told AP in a telephone interview that "this is a message to the Israelis that all their walls and fences cannot prevent us from infiltrating Israel and doing whatever we want."

Abu Qusay said the militant groups were trying to carry out more devastating attacks. "We planned this attack to be a major one, to target their infrastructure, to show them that not only can we kill you, but destroy your infrastructure as well, like you destroy ours," he said.

So far, more than 100 Palestinian suicide bombings since 2000 have targeted Israeli malls, cafes and buses, but no single attack has claimed more than 30 lives.

Palestinian militants in Gaza have been stepping up attacks in recent weeks, in response to Sharon's proposal to withdraw from all or most of Gaza if peace talks with the Palestinians remain frozen. The militants want to claim victory by creating the impression that they are driving Israel out of Gaza.

Israel has also escalated its military activity in Gaza, for fear that a withdrawal could be perceived as a sign of weakness.

In a first response to the bombing, Sharon canceled a summit, tentatively set for Tuesday, with his Palestinian counterpart, Ahmed Qureia.

Early Monday, Israeli helicopters fired missiles at two suspected Hamas weapons workshops in Gaza City, causing some damage, but no injuries. Sharon was awaiting the return of Defense Minister Shaul Mofaz from a U.S. trip later Tuesday before deciding on possible larger-scale strikes, a security official said.

Israel closed the Erez crossing, barring some 19,000 Gaza workers from jobs in Israel. Police also stepped up security at all Israeli seaports, airports and train stations. Amos Ron, director of Israel's Ports Authority, said bombing-sniffing dogs would begin patrolling Ashdod port in the coming days.

Palestinian militants, meanwhile, fired an anti-tank missile at an armored bus carrying Israeli settlers in Gaza early Monday, the army said. The missile tore through the bus, causing heavy damage, but no one was hurt, the army said.

Sunday's bombing was the fourth attack carried out jointly by Al Aqsa and Hamas in recent weeks. The bombers, Mahmoud Salem and Nabil Massoud, attended the same mosque. One of their relatives initially said they were in the same class, but it turned out they attended different schools. Their fathers described them as devout Muslims and said they were proud of their sons.

Salem was recruited by Hamas and Massoud by Al Aqsa.

Sometime after 4:30 p.m. local time they reached Ashdod port, about 14 miles north of Gaza. One entered the port and blew himself up near a group of workers standing outside a machine repair workshop. The second bomber detonated his explosives near the perimeter fence of the port.

"When I arrived here I saw body parts strewn around, some of them hanging on the barbed wire," said witness Moshe Rima, who works for a company in the port.

Ten people were killed, in addition to the bombers, and 18 were wounded, the Israeli Foreign Ministry said. Officials initially said 11 Israelis were killed. Israeli security officials said the bombers apparently used plastic explosives.

Palestinian militants had previously targeted a fuel depot, in 2002, but that attack did not kill anyone. A plot to blow up skyscrapers in Tel Aviv was foiled.